Automatic film processors in commercial settings typically develop film exposed to various forms of energy ranging from X-ray to visible light. The energy is used to expose the film and create a latent image on the film which becomes visible during development. In the development process, the film is first immersed in developer solution. The film is thereafter passed through a fixer solution which preserves the visible image. The film is subsequently washed and dried by the processor so that it is available for viewing and analysis.
Such film processors, however, are susceptible to various difficulties arising from performance fluctuations during operation, particularly over extended periods of time. Principal among these difficulties is deteriorated or spent developer solution in the processor. The developer solution at full strength is typically yellowish in color; however, the coloration of the solution becomes dark brown over time due to oxidation of the developer solution. Inasmuch as quality of the visible image degrades over time as a function of aged developer solution, the need exists to accurately determine the developer solution strength. Deteriorated or weak processor chemicals affect the quality of the films developed and can result in reexposure of additional film. In the case of X-ray film processing, degraded developer solution frequently requires further patient exposure to radiation doses.
Conventional wisdom therefore dictates implementation of quality control programs to regularly check the film processor. Such programs typically use a quantity of film for sensitometric testing of the processor. A sensitometer exposes a test strip of film with a preselected light source. After the exposed strip is processed, a desensitometer is used to measure the various strips on the test film. This is accomplished by shining light through an image area on the film and measuring the amount of light passed through. These optical density values are then plotted and compared to data supplied from the film manufacturer to ensure proper development of the film.
Various types of self-cleaning X-ray film processors have also been proposed to deal, at least in part, with the problem of deterioration of developer solution, such as in Burbury U.S. Pat. No. 4,650,308, incorporated by reference herein. These processors employ a sequential cleaning cycle to periodically purge the various compartments of a film processor, including developer and fixer compartments. Thus, the developer strength is maintained through systemized cleaning and replenishment of the chemicals. However, these systems fail to adequately deal with monitoring of the developer solution in real time.
Other X-ray film processors have dealt with the problem of automatic exposure control, such as in U.S. Pat. No. 4,679,217. Adjustment of exposure control enables the X-ray technician to compensate for film density at the time of exposure when it is known that the developer strength of the processor has been decreased, but as of this time that has been essentially guess work.